Due to their superiority in processability, chemical resistance, weatherability, electrical properties, mechanical strength, etc., thermoplastic resins have so far been used extensively not only in the field of industrial and domestic electrical products but also in various fields including buildings, interior decorative articles and automotive parts, and the applications in which thermoplastic resins are used have been expanding. With the expansion of applications, the thermoplastic resins also have come to be required to have flame retardancy and the performance requirement has been becoming severer year by year. Recently in particular, a flame-retardant resin composition comprising a thermoplastic resin and a halogen-containing compound or a flame-retardant resin composition comprising a thermoplastic resin, a halogen-containing compound and an antimony oxide, which compositions each are a main current in the conventional flame retardation technology, are regarded as defective because they generate a halogenous gas during combustion or molding. Accordingly, a flame-retardant resin composition which does not generate such a halogenous gas during combustion or molding has come to be desired.
In order to meet these requirements, a flame-retardant resin composition containing ammonium polyphosphate and one or more nitrogen-containing organic compounds which generate nonflammable gases (water, carbon dioxide, ammonia, nitrogen, etc.) and a carbonaceous residue upon pyrolysis has been proposed from recent investigations. For example, a flame-retardant composition comprising a polymer or oligomer of a 1,3,5-triazine derivative and ammonium polyphosphate is proposed in JP-A-59-147050 (corresponding to U.S. Pat. No. 4,504,610) (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and EP-A-0475418; a flame-retardant composition comprising ammonium polyphosphate and a reaction product of a nitrogen compound having a &gt;C.dbd.O, &gt;C.dbd.S or &gt;NH group bonded to a cyclic structure with an aldehyde is proposed in JP-A-52-146452 (corresponding to U.S. Pat. No. 4,193,945); a flame-retardant composition comprising ammonium polyphosphate and a product of the reaction of benzylguanamine with an aldehyde is proposed in JP-A-55-129435 (corresponding to European Patent 14463); and a flame-retardant composition comprising an isocyanuric acid derivative and ammonium polyphosphate is proposed in JP-A-54-53156 (corresponding to U.S. Pat. No. 4,198,493).
Further, as a combination containing no nitrogen compound, JP-A-64-14277 (corresponding to U.S. Pat. No. 4,871,795) proposes a flame-retardant composition comprising a high-viscosity silicone oil, a silicone resin, a polyol and an ammonium phosphate.
However, although the conventional flame-retardant compositions enumerated above have high flame retardancy, the ammonium polyphosphate in the compositions is so subject to hydrolysis because of the chemical structure thereof that molded materials obtained from the compositions under high-temperature and high-humidity conditions, e.g., in the rainy season, undergo a phenomenon in which the ammonium polyphosphate severely bleeds to the surfaces of the molded materials due to its hygroscopicity, solubility in water, and hydrolyzability. Further, the flame-retardant resin composition described in EP-A-0475418 has a drawback that since the nitrogen-containing organic compound contained therein has hygroscopicity, the ammonium polyphosphate is subject to hydrolysis and the hydrolyzate is apt to bleed to the surface of a molded material obtained from the composition and, as a result, the molded material comes to have a greatly reduced electrical surface resistance and is hence unusable as an electrical insulating material or the like under high-temperature and high-humidity conditions.